We are upgrading the windows to european non-flanged and the house currently has sheathing and tar paper and cedar boards

My research so far points to using some sort of liquid flashing to seal the opening and create a continuous sill to the tar paper.

What I can’t find is any examples of how to connect the exterior of the window to the liquid flashing. There doesn’t seem to be any tape that is approved to stick to FastFlash. Everyone seems to just use flanged windows and roll on another layer of liquid flashing. 
European window examples do not use liquid flashing so the tape examples stick to other substrates with no issues

Am I going about this the wrong way? I’m not sure what else to do, maybe just use fentrim or wigluv (with the Dockskin primer?)

Or leave the exterior side open on the sides and bottom, just add head flashing and seal it with fastflash
The backer rod would be exposed on the sides and the bottom. I’m not sure if that’s a big deal. Any water would run down the sides and off the liquid flashing and on to the tar paper

FastFlash is what’s available locally but I think Tyvek LiquidArmor LT would work the same I just can’t find it absolutely anywhere. Maybe someone has sources for BC lower mainland and if it’s any cheaper even

Or any other products for that matter. 
I’ve considered Dymonic 100 but that’s even more unreliable as to what would stick to it. The curing time seems longer also